A gas sensor is described in German Published Patent Application No. 41 26 378. The gas sensor contains an oblong, planar sensor element, which makes contact with the gas to be analyzed, using a section on the side of the gas to be analyzed. Contact surfaces for contacting the sensor element to evaluation electronics arranged outside of the gas sensor are provided on a terminal-side section of the sensor element facing away from the section on the side of the gas to be analyzed. The measuring-gas-side and terminal-side sections of the sensor element are separated by a seal arrangement. The seal assembly is arranged between a molded ceramic part on the side of the measuring gas and a molded ceramic part on the side of the terminal. The molded ceramic parts and the seal assembly contain openings for accommodating the sensor element. To produce the seal, a premolded sealing ring is initially pressed between the two molded ceramic parts. While the sealing ring is pressed, it is converted to steatite powder, which then rests radially against the sensor element and the housing and thereby seals the sensor element in the housing. A seal assembly is also described in German Published Patent Application No. 195 32 090, where a further boron-nitride sealing element is pressed between two steatite sealing elements. As in German Published Patent Application No. 41 26 378, the sealing action may be attained in that a force is exerted by the molded ceramic parts on the sealing material in such a manner, that the sealing material is markedly compressed and presses against the sensor element and the housing due to deformation.
In the case of the described seal assemblies, it may be undesired that the sealing material is required to be under constant high pressure to achieve the required sealing effect. Therefore, such seal assemblies may be expensive and difficult to produce. In addition, the sensor element may be damaged by the forces occurring while the seal assembly is being pressed.
European Published Patent Application No. 0 706 046 describes a seal assembly, in which the sensor element is fixed in position in a ceramic retaining body by a glass seal. In addition, German Published Patent Application No. 198 52 674 and German Published Patent Application No. 101 23 168 describe a seal assembly having a sealing element, which contains a mixture of a ceramic powder, e.g. steatite, and a glass powder. The sealing element is heated to a maximum temperature of 1000 degrees Celsius. Due to this heat treatment, the glass powder is melted in the steatite matrix and thus diffuses into the pores of the steatite matrix. The ceramic portion is not sintered by the heat treatment.
In the case of seal assemblies that contain a glass fraction, it may be undesired that states of stress, which may lead to tears in the glass-containing seal, may occur during operational heating or cooling of the seal assembly, or under stress due to thermal shock. In addition, the electrical resistance of a glass-containing seal decreases considerably in response to higher temperatures occurring during the operation of the gas sensor so that unwanted currents may occur between the sensor element and the housing.